1. Field of Invention
This invention relates to data transmission over wavelength division multiplexed passive optical networks. More specifically, the invention relates to providing a bandwidth efficient way of delivering multiple broadcast services on such a network using broadband spontaneous emission optical sources.
2. Description of Related Art
Wavelength division multiplexed passive optical networks (WDM PON) can be used to deliver both switched and broadcast services on the same fiber plant. P. P. Iannone, K. C. Reichmann and N. J. Frigo, xe2x80x9cHigh-Speed Point-to-Point and Multiple Broadcast Services Delivered over a WDM Passive Optical Networkxe2x80x9d, IEEE Photonics Technology Letters, Vol. 10, No. 9, pp. 1328-1330, September 1998. As shown in FIG. 1, to deliver broadcast services, a conventional WDM PON system 100 includes at least one broadband optical source 110, at least one WDM remote node 120 and a plurality of user nodes 130-150. The broadband optical source 110 is coupled to the input port of the WDM remote node 120. The broadband optical source 110 is modulated to incorporate the service data to be broadcast to the user nodes 130-150. The plurality of user nodes 130-150 are coupled to the various outputs of the WDM remote node 120.
The WDM remote node 120 consists of a wavelength router for distributing data to different user nodes. A wavelength router is also called an arrayed waveguide router (AWG) or waveguide grating router (WGR). The wavelength router has a cyclical property. For example, given a four-output port wavelength router, if the sum of equally separated input wavelengths xcex1, xcex2, xcex3, xcex4 are coupled to its input port, then xcex1 will appear at output port 1, xcex2 at output port 2 etc., in similar ways as any other wavelength demultiplexers. However, if wavelengths xcex5, xcex6, xcex7, xcex8 are coupled to the input port, then xcex5 will again appear at output port 1, xcex6 at output port 2 etc. In other words, the wavelength demultiplexing property repeats over ranges of wavelengths or frequencies. The smallest range of wavelength over which the cyclical property repeats is called the free spectral range (FSR) of the wavelength router. In broadcast operation, the output spectrum of the broadband optical source 110, modulated with the broadcast data, contains wavelength components covering at least one FSR of the wavelength router. The wavelength router slices the frequency spectrum of the optical signal produced by the optical source 110 to deliver the broadcast service to the user nodes 130-150. Each user node 130-150 gets part of the broadcast signal, albeit at different wavelengths. For example, as shown in FIG. 1, one FSR of the wavelength router is shown. However, each node normally sees multiple FSR""s and the spectrum of the optical source 110 usually covers more than one FSR. User node 130 receives a bottom slice of the FSR, user node 140 receives a middle slice of the FSR and user node 150 receives a top slice of the FSR. In actuality, the cyclical property of the wavelength router comes from the fact that it is implemented as an interferrometric device.
Multiple broadcast services have also been realized using different FSRs of the wavelength router. FIG. 2 illustrates a different conventional system architecture for providing multi-band broadcast services in a WDM PON. Specifically, as shown in FIG. 2, the system 200 includes a first optical source 205 and a second optical source 210, a combiner 215, a WDM remote node 220 and a plurality of user nodes 230-250. The output from the first and second optical sources 205, 210 are coupled to the combiner 215, which is also coupled to the input of the WDM remote node 220. The plurality of user nodes 230-250 are also coupled to the various outputs of the WDM remote node 220. The WDM remote node 220 also incorporates a wavelength router, which has the wavelength or frequency cyclic property, for distributing different broadcast data streams within specific portions of the frequency spectrum to various user nodes. As shown in FIG. 2, the illustrations of user nodes 230-250 each include a graph depicting the slices of the transmitted spectrum received by each user node.
The first optical source 205 generates the optical signal associated with a first frequency band, B1, which encompasses a specific FSR of the router within the WDM node 220. This optical signal is modulated by the data of the first broadcast service. The second optical source 210 generates the optical signal associated with a second frequency band, B2, which encompasses the next FSR of the router within the WDM node 220. This optical signal is modulated by the data of the second broadcast service. Therefore, as shown in FIG. 2, user node 230 receives low end slices of both the first and second frequency bands B1 and B2. Similarly, user node 240 receives intermediate slices of both the first and second frequency bands B1 and B2. User node 250 receives high end slices of the first and second frequency bands. Although FIG. 2 depicts only two FSR""s B1 and B2, it should be appreciated that any number of free spectral ranges may be used to transmit different data streams and to support either broadcast or switched services. Switched services will not be described in this document. Although FIG. 2 illustrates only three user nodes 230, 240, 250, it should also be appreciated that the number of user nodes is only limited by the number of output ports of the wavelength router in the remote node 220.
In such broadcast applications, the optical sources 110, 205 and 210 are broadband sources. Examples of broadband sources include multi-wavelength laser diodes and spontaneous emission sources such as light emitting diodes (LED""s) or Erbium doped fiber amplifiers (EDFA""s) operated in spontaneous emission mode. The optical sources 110, 205 and 210 are located in a hub station or a central office of the service provider, which is also called a Host Digital Terminal (HDT) sometimes, and the transmitted signals are connected to the input port of the remote nodes 120 and 220 through a section of feeder optical fiber.
However, there is a need for producing a system for delivering multiple broadcast services in a spectral efficient way and a scalable way so that the network services can grow easily. Therefore, the invention provides a method and system for delivering multiple broadcast services in a bandwidth efficient and easily growable way in a WDM PON using broadband spontaneous emission optical sources.
The invention relates to using a chirped fiber Bragg grating (FBG) coupled to an optical circulator in a transmitter and/or a receiver used in a WDM PON to select one or more FSR""s to be delivered to user nodes. Such an arrangement can be cascaded to combine or separate different sections of the frequency spectrum. In accordance with the exemplary embodiments of the invention, a WDM PON may include multiple transmitters at the hub or central office and multiple receivers located at different user nodes within the PON.
In accordance with the exemplary embodiments of the invention, a filter comprised of an optical circulator and a chirped FBG confines the output from a spontaneous emission source to a desired spectral region. The hub station or central office utilizes such cascaded filters to combine multiple non-overlapping optical spectra from transmitters of different broadband services to be delivered through the feeder optical fiber. At a user node, a band splitter utilizing such cascaded filters separates the combined broadcast spectrum into individual service bands received by separate receivers.
The invention may be implemented to separate the spectral output of a spontaneous emission source such as an EDFA or a praseodymium-doped fiber amplifier (PDFA) into different bands for supporting different broadcast services, allowing sharing the cost of a single optical source.
The invention is also useful in situations in which a relaxed tolerance is required for the source wavelengths, e.g. using an uncooled laser in a WDM system without spectral slicing, provided the wavelengths are separated far enough from each other. Specifically, the broadband chirped FBG/optical circulator wavelength add/drop technique is useful in coarse WDM systems which allow for more tolerance to wavelength shifting.